Surface-emitting laser diode

ABSTRACT

A tunable [tunable] surface-emitting laser diode in which in the manner of the TTG laser diode an active layer (5) and a tuning layer (3) are arranged transverse to one another between contact layers (2, 6) and are separated from one another by a central layer (4), with the result that given suitable doping of the surrounding semiconductor material these layers can be driven separately, and in which above a region provided for generating radiation the semiconductor material has on the surface a spatial periodic structure which is provided with a thin metallic film (7), with the result that during operation of the laser diode radiation is emitted directionally from the surface by means of exciting surface plasmon polaritons, and that this emission direction can be varied by tuning the wavelength of the radiation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface-emitting laser diode in whichdifferent emission directions can be realized.

2. Description of the Related Art

European Patent Document EP-A-0 442 002 describes a radiation-generatingsemiconductor component in which there are present on the surface of thesemiconductor material spatial periodic structures which are coated witha thin metallic film. Surface plasmon polaritons are excited on thesurface of this metallic film by photons generated in the active layer.An extremely focused emission from the surface is achieved by thisexcitation of surface modes. Surface plasmon polaritons are transverseelectric (TE) or transverse magnetic (TM) surface modes which canpropagate at the interface between two different media. The excitationof these modes requires a suitable periodic structure of the interface,which is formed in the case of the component described by a spatialstructure, produced by etching, for example, of the semiconductorsurface. The loss mechanisms which occur in conventionalradiation-generating components and limit efficiency can be avoided, agreatly reduced line width of the emitted light and a drastic increasein the external quantum efficiency being achieved simultaneously. Theemission takes place, in addition, with a defined polarization directionwhich is fixed by the arrangement and alignment of the periodic surfacestructure.

European Patent Document EP-A-0 360 011 describes a tunable DFB laserdiode in which the active layer and the tuning layer are arrangedtransversely relatively to one another and can be driven separately viaan intermediate layer which is located therebetween. This laser diode,known to experts as a TTG-DFB laser diode, has come to exist in themeantime in various modifications which relate, in particular, tovarious arrangements of the contacts required for the supply leads. Suchdiodes can be constructed on conductive and on semi-insulatingsubstrates. The layer structure can be adapted in detail to therespective requirements, in particular in the case of the production ofhighly integrated components.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a surface-emittinglaser diode which emits in an extremely focused fashion and whoseemission direction can be varied without varying the structural designor the arrangement of the diode.

This and other objects and advantages of the invention are achieved bymeans of the tunable laser diode having two layers which are separatedby a central layer, are arranged parallel, and with reference to theplanes of the layers, transverse to one another and are made fromsemiconductor materials having different energy band gaps, and of whichone is provided as active layer and one as tuning layer and which areconnected in an electrically conductive fashion to contacts in such away that a separate current injection into the active layer and into thetuning layer can be performed, having a spatial periodic structure whichis present in a region, arranged with reference to the planes of thelayers in a fashion transverse to a region provided for generatingradiation in the active layer, of the surface of an outermostsemiconductor layer and is covered at least partially with a metallicfilm, the height of this structure and the length of in each case oneperiod of the spatial periodic structure, the minimum distance of themetallic film from the active layer, and the thickness of the metallicfilm being dimensioned such that during operation of the laser diodesurface modes are excited by photons generated in the active layer onthe surface of the metallic film averted from the active layer, andhaving measures for achieving a laser resonance. Further embodiments ofthe invention provide that the measures for achieving a laser resonancecomprise a DFB (distributed feedback) grating which is arranged paralleland with reference to the planes of the layers transverse to the activelayer. Alternately, or additionally, the measures for achieving a laserresonance comprise reflecting end faces, arranged perpendicular to theplanes of the layers, on two mutually opposite edges of the activelayer. The measures for achieving a laser resonance comprise at leastone reflective coating arranged parallel and with reference to theplanes of the layers transverse to the active layer.

In a preferred embodiment, the metallic film is covered by a dielectriclayer. The present invention achieves the object by the combination of alayer structure resembling the TTG laser diode with a surfaceconfiguration which permits the excitation of surface modes. This laserdiode according to the invention is described below with the aid ofvarious exemplary embodiments which are represented in FIGS. 1 to 6.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 respectively show an embodiment of the laser diodeaccording to the invention, in cross section;

FIG. 4 shows a longitudinal section of the embodiment represented inFIG. 3; and

FIGS. 5 and 6 respectively show sectional top views of the laser diodesof FIGS. 1 and 3, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the laser diode according to the invention, a tuning layer isprovided transverse to an active layer. This tuning layer isdistinguished from the active layer by the energy band gap of thesemiconductor material used. It is achieved as a result that noradiation is generated in the tuning layer. The active layer and thetuning layer are arranged between doped semiconductor layers, thesesemiconductor layers being doped in such a way that currents can beinjected separately into the active layer and into the tuning layer. Forthis purpose, it is the case, for example, that a central layer arrangedbetween the active layer and the tuning layer is doped for oneconductivity type, while two contact layers arranged respectively on theopposite side of the active layer and of the tuning layer are doped forthe opposite conductivity type. If this central layer and the twocontact layers are respectively connected separately from one another,for example via a conductively doped semiconductor material, to onecontact each, controllable currents can be applied independently of oneanother to the active layer and to the tuning layer by applying suitablepotentials to these contacts. Consequently, the wavelength of theradiation generated in the active layer can be tuned. The spatialperiodic structure is arranged in the surface of the semiconductormaterial above the region provided for generating radiation in theactive layer. It is then possible for surface modes which lead toemission of light from the surface to be excited on the outside of thethin metallic film, applied to surface, during operation of the laserdiode. The emission direction is a function in this case of thewavelength of the light and of the periodic length of the structure. Itis therefore possible, for a given periodic length, to vary the emissiondirection by changing the wavelength. A layer sequence which is shown incross section is applied to a conductive substrate 1 in FIG. 1. Thefirst step is to apply a lower contact layer 2. Located thereupon insequence are the tuning layer 3, the central layer 4 and the activelayer 5. An upper contact layer 6 contains a DFB distributed feedbackgrating 8. This contact layer 6 is covered by a cover layer 10 whosesurface has the spatial periodic structure. The DFB grating 8 serves thepurpose of producing laser resonance. Instead of this DFB grating 8, orin addition thereto, it is possible for there to be present at thelongitudinal boundaries of the active layer 5 reflecting end faces towhich reflective coatings may have been applied for silvering purposes,the result being that the laser resonator is a Fabry-Perot laser. Thelayer structure in FIG. 1 is bounded in strip form and laterally bysemiconductor material. In this case, the cut extends transverse to thelongitudinal direction of the strip. Located in the cover layer 10 is atwo-dimensionally arranged periodic spatial structure as representedsimilarly in an oblique top view in FIG. 5. A thin metallic film 7,which is preferably silver, gold or aluminium, for example, is locatedon this structure. The periodic structure can also be present directlyin the upper contact layer 6. The cover layer 10 is then superfluous. Itis likewise possible for the arrangement of an active layer 5 and tuninglayer 3 to be exchanged, that is to say the tuning layer is thenarranged above the active layer with respect to the substrate 1. A DFBgrating 8 can also be arranged at a different location within the layersequence. A grating is superfluous in the presence of reflecting endfaces. However, a grating can be present in addition or instead thereof,for example as a DBR distributed Bragg reflector grating. The lowercontact layer 2 can likewise be eliminated when the current injection isperformed through a conductive substrate 1. The thickness of themetallic film 7 is typically between 0.01 μm and 0.1 μm. The contacts11, 12, 13 on the upper side or on the underside, averted from the layersequence, of the substrate 1 are respectively applied to dopedsemi-conductor materials. In the exemplary embodiment of FIG. 1, thecontact 11 is located on the semiconductor material connected in anelectrically conductive fashion to the upper contact layer 6. The regionwhich is arranged laterally relative to the strip having the layersequence and to which the contact 12 is applied, and the central layer 4are semiconductor material doped oppositely thereto. Just like thesubstrate 1, the lower contact layer 2 has a doping with the same signas the upper contact layer 6. In order to avoid parasitic currents,additional insulation regions or blocking pn junctions, which areomitted in FIG. 1 for the purpose of clarity, can be arranged at variouspoints having undesired pn junctions. In the case of the exemplaryembodiment of FIG. 1, it is possible in the material system of GaAs, forexample, for the substrate 1 to be GaAs, for the contact layers 2, 6 andthe central layer 4 respectively to be AlGaAs, and for the tuning layer3 and the active layer 5 respectively to be GaAs. The active layer 5 canalso be a quantum well structure having a sequence of thin layers ofdifferent energy band gaps: in the exemplary embodiment, for example,layers alternately of GaAs and InGaAs. The cover layer 10 which ispossibly present is likewise AlGaAs. The substrate 1 and the contactlayers 2, 6 (possibly also the cover layer 10) are doped, for example,in a p-conducting fashion. The central layer 4 is doped in ann-conducting fashion in that case. Possible compositions are, forexample, (Al₀.35 Ga₀.65)As for the central layer 4 and, for example,(Al₀.3 Ga₀.7)As for the contact layers 2, 6. Typical layer thicknessesare: 1000 nm, for example, for the lower contact layer 2, 250 nm, forexample, for the tuning layer 3,500 nm, for example, for the centrallayer 4, 90 nm, for example, for the active layer 5, and 400 nm to 1300nm, for example, for the upper contact layer 6 in the exemplaryembodiment the cover layer. The typical dimensions, illustrated in FIG.4, of the grating-type structure of the surface are, for example, 1300nm for the length Lg of the grating period, 100 nm for the height h ofthe structure (that is to say, the difference between the maximum andminimum distance of the metallic film 7 from the active layer 5), 25 nm,for example, for the thickness d of the metallic film 7, and between 100and 500 nm, for example, for the thickness of a dielectric possiblyapplied to the metallic film 7. The resonator length, that is to say thedistance from reflecting end face to reflecting end face, is 500 μm, forexample, and the length of the structured region of the surface in thelongitudinal direction of the resonator is 100 μm, for example.

FIG. 2 shows an alternative embodiment, in which reflective coatings 19,20 are arranged for the purpose of forming a vertical laser resonancerespectively above and below the contact layers 2, 6. The upperreflective coating 20 can be eliminated if the metallic film 7 has anadequate reflective effect. As in FIG. 1, the lower contact layer 2, thetuning layer 3, the central layer 4, the active layer 5 and the uppercontact layer 6 are arranged one above another between the reflectivecoatings 19, 20. Various emission directions which are possible bytuning the wavelength are indicated by the arrows. In addition,detectors D1, D2, . . . Dn, which are arranged in various emissiondirections, are illustrated in FIG. 2. Instead of the detectors, it ispossible to provide glass fibers or connections for glass fibers intowhich the light emitted by the laser diode can respectively be directed.In principle, all the modifications as known from the TTG laser diodecome into consideration for the part of the laser diode according to theinvention which is provided for the tuning. As an example, FIG. 3 showsan arrangement in which all the contacts are arranged on the upper side,and the substrate 1 is a semi-insulating material. Furthermore, by wayof example in the case of this laser diode a simplified design isrepresented in which the layer sequence comprises only the contact layer2, the tuning layer 3, the central layer 4, the active layer 5 and thecontact layer 6. The spatial periodic structure is located directly inthe surface of this upper contact layer 6. As an alternative to thetwo-dimensional structure of FIGS. 1 and 2, a one-dimensional structureis shown here which has a periodic sequence in the longitudinaldirection of the laser strip. Such a structure is represented in adetail in FIG. 6 in a perspective top view.

A longitudinal section belonging to FIG. 3 is represented in FIG. 4. Thelength Lg of the period of the structure, the height h of this structureand the minimum distance a of the metallic film 7 from the active layer5 are illustrated in FIG. 4. Various emission directions are indicatedonce again by the arrows above. Reflective coatings are provided asreflecting end faces 9 in this laser diode.

In order to permit a separate supply of power to the active layer 5 andto the tuning layer 3, there is embedded in the semi-insulatingsubstrate 1 a feed layer 14 which, for example, is produced byimplanting dopant before the overgrowing. The contact 11 is locateddirectly on the doped contact layer 6. The lower contact layer 2 isdoped for the same type of conduction, as are the feed layer 14 and thelateral region 17 applied thereon. A contact 18 is located on thislateral region 17. The connection of the oppositely doped central layer4 is performed via the correspondingly doped second lateral region 16,to which the contact 12 is applied. In the case of the use of asemi-insulating substrate, as well, all the modifications which areknown from TTG laser diodes on a semi-insulating substrate are availableindividually for the laser diode according to the invention. Thus, inparticular, the laser diode according to FIG. 3 can be supplemented by aDFB distributed feedback grating or by reflective coatings forconstructing a vertical resonator. All the embodiments known to theperson skilled in the art from the cited European Patent Document EP-A-0442 002 come into consideration for the periodic structure of thesurface. This structure is additionally illustrated by FIGS. 5 and 6 forthe embodiments described with the aid of FIGS. 1 to 4. The periodicstructure can be formed by trenches which are aligned parallel to oneanother and which, in the case of a laser diode having a horizontalresonator, do not extend in the propagation direction of the light inthe resonator. The structure can be periodic in all directions in theplane of the active layer.

This is the case, for example, for a cruciform arrangement of trenchesor webs. Two families of trenches or webs arranged parallel to oneanother and respectively at the same distances from the adjacenttrenches or webs are, for example, arranged perpendicular to oneanother. What are then to be regarded as the periodic length in thesense of the claims are the dimensions which are characteristic of theproperty of the surface modes, that is to say the lengths of theperiodicity in the two directions which are respectively perpendicularto a family of trenches or webs. The DFB grating 8 is also illustratedin FIG. 5, the frontal view perpendicular to the plane of the drawing inFIG. 5 corresponding to the frontal view perpendicular to the plane ofthe drawing in FIG. 1. The frontal view of FIG. 6 is a detail from FIG.4. The direction of view denoted by the arrow in FIG. 6 delivers acorresponding detail from FIG. 3.

The relevant possibilities remain open for structuring the active layer5 and the tuning layer 3. In particular, the tuning layer 3 can have aquantum well structure.

In order to excite surface modes of higher order (surface plasmonpolaritons), the metallic film 7 can be covered by a dielectric layer.

In the case of a laser diode having a strip-shaped horizontal resonatorby means of which a propagation direction of the light is fixed in theplane of the active layer, the emission results owing to excitation ofsurface modes in the plane which is fixed by this propagation directionand the perpendicular to the plane of the layer, if the structure of thesurface is periodic primarily in the propagation direction. This is thecase, for example, for structuring by trenches or webs which extendperpendicular to the longitudinal direction of the resonator. If thestructure of the surface, by contrast, is similar but turned, theemission then takes place in that plane which is fixed by theperpendicular to the plane of the layer and by the direction of theshortest periodicity of the structure, that is to say in acorrespondingly turned plane. Consequently, it is possible by aligningthe structure of the surface to undertake an alignment of the emissiondirections, which can be controlled by tuning the wavelength, relativeto the laser diode. The focusing of the radiation which is emitted uponexcitation of surface modes is so good that it is possible for aprecisely defined emission direction to be maintained very well usingthe laser diode according to the invention. Even a slight change in thewavelength, that is to say by means of a slight change in thecorresponding control voltage, enables a marked change in the emissiondirection. Small voltage changes therefore cause clearly measurableangular variations in the emission direction. It is thus possible torealise in a simple way a light-emitting component having anelectrically variable emission angle. Various emission directions can berealised using the same laser diode, without the need to takeappropriate precautions for each emission direction in the structuraldesign of the laser diode, or even without the need to turn the laserdiode as a whole.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

We claim:
 1. A tunable surface-emitting laser diode, comprising:acentral layer; two layers separated by said central layer, said twolayers being arranged parallel, and with reference to planes of said twolayers, transverse to one another and being made from semiconductormaterials having different energy band gaps, and of which one isprovided as an active layer and one as a tuning layer; contactsconnected in an electrically conductive fashion to said two layers insuch a way that a separate current injection into the active layer andinto the tuning layer can be performed; a semiconductor layer on saidtwo layers and having a surface defining a spatial periodic structurewhich is present in a region, arranged with reference to the planes ofsaid two layers in a fashion transverse to a region provided forgenerating radiation in the active layer; a metallic film at leastpartially covering said spatial periodic structure; said spatialperiodic structure being of a height and a length of each period of saidspatial periodic structure, a minimum distance of said metallic filmfrom the active layer, and a thickness of said metallic film beingdimensioned such that during operation of the laser diode surface modesare excited by photons generated in the active layer on a surface ofsaid metallic film averted from the active layer, and means forachieving laser resonance during operation of said laser diode, saidmeans for achieving a laser resonance comprising at least one reflectivecoating arranged parallel and with reference to the planes of said twolayers transverse to the active layer.
 2. A tunable surface-emittinglaser diode as claimed in claim 1, wherein said means for achievinglaser resonance comprise a DFB grating which is arranged parallel andwith reference to the planes of said two layers transverse to the activelayer.
 3. A tunable surface-emitting laser diode as claimed in claim 1,further comprising:a dielectric layer covering said metallic film.